Depilation with a solution of an inorganic phospho-sulfur compound



United States Patent 3,365,265 DEPILATION WITH A SOLUTION OF AN INOR- GANIC PHOSPHO-SULFUR COMPOUND Thomas C. Thorstensen, Chelmsford, Mass, and James H. Brown, Lewiston, and John C. Pernert, Niagara Falls, N.Y., assignors to Hooker Chemical Corporation,

Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Nov. 30, 1962, Ser. No. 241,132

16 Claims. (Cl. 8-94.16)

This invention relates to improvements in the processing of animal hides into leather and more particularly relates to improved compositions and methods for the unhairing of the hides during such processing.

In the processing of animal hides or skins into leather, certain preliminary operations are customarily utilized in order to place the hide in suitable condition for the ultimate tanning operation. Initially, once the hide has been removed from the animal, it is necessary to prevent decay of the hide by the bacteria and autolytic enzymes contained in the hide. This is eifected by a curing operation. This curing operation may be carried out in any one of a number of ways although, most frequently, it is done by packing the hide in rock salt. The thus-cured hide is then transported to the tannery where it is stored until such time as it can be put through the pre-tannage or so called beam-house operations.

From the tannery storage, the cured hides undergo a soaking operation wherein the hides are contacted with water over an extended period of time, generally of 24 hours or more. The purpose of this operation is to remove any salt, dirt and soluble protein matter from the hides as well as to allow the skin fibers of the hides to absorb water and swell. Generally, during the soaking operation, the hides are removed from the water and are fleshed, i.e., the flesh is removed from the hide, thus allowing the skins to hydrate better so as to expedite the soaking and subsequent operations. After the fleshing operation, the soaking process is completed and the hides are in condition for the unhairing process.

In this process, the hides are subjected to the action of some material which will loosen the hair on the hides so that it may be easily removed. In this regard, it must be noted that although it is desirable that the unhairing operation be carried out as rapidly as possible, the materials used for unhairing should not have any deleterious effect on either the hide itself or on the hair which is removed. The reason for avoiding destruction of the latter is that the hair is a valuable by-product which is desirably recovered and sold. The reason for avoiding destruction of the former is, of course, obvious.

In the past, the unhairing of the hides has been carried out by a prolonged contact of the skin with a saturated lime solution containing excess solid lime. While this method is effective in removing the hair from the hides, without excessive damage to either the hide or the hair, the process is quite time-consuming, generally requiring 2 or 3 days or longer to complete the unhairing operation. For this reason, it has heretofore been proposed to add various other reagents to the lime solution in order to speed up the unhairing operation. For this purpose, various sulfides, such as arsenic sulfide, sodium sulfide, sodium sulfhydrate, and the like, have been utilized. Although such materials do effect a material reduction in the time required to unhair, great care must be exercised in their use to prevent damage to the hair. In this regard, it has been found that where an excess of these materials is added to the lime solution or slurry, considerable hair damage results. While various other reagents have been proposed for use with lime as substitutes for the above sulfides, such as dimethylamine, these materials have 3,365,265 Patented Jan. 23, 1968 generally been quite expensive and therefore have not been widely used.

It is an object of the present invention to provide an improved composition of matter which may be utilized with lime in the unhairing of animal hides.

Another object of the present invention is to provide,

a composition for use in the unhairing of animal hides which composition, in conjunction with lime, effects a rapid unhairing, without appreciable damage to the hair.

A further object of the present invention is to provide an improved process for the unhairing of animal hides which process may be carried out rapidly without damaging either the hide or the hair which is removed.

These and other objects will become apparent to those,

which excess has, in the past, resulted in damage to or even complete destruction of the hair. The presence of this controlled amount of sulfhydrate in the unhairing solution markedly increases the unhairing action of the lime solution or slurry and thus reduces the time required to effect unhairing.

When a solution or slurry of lime containing the inorganic phospho-sulfur compound containing at least, one P-S bond of the present invention is utilized in unhairing hides or skins, there is a hydrolysis of the phosphorus and sulphur compound which results in sulfhydrate and phosphate ions. The rate of hydrolysis of the phosphorus and sulphur compound is apparently of the same magnitude as the rate of consumption of the sulfhydrate ions by the unhairing reaction. The formation of the sulfhydrate and phosphate ions, by the hydrolysis of the phosphorous and sulphur compound contributes to the acidity in the unhairing solution. Inasmuch as the lime solution used is generally a slurry of lime, this acidity effects a further dissolving of more calcium hydroxide. The resulting increased calcium concentration in the solution then results in a lowering of the pH of the solution, due to a common ion effect in the solubility product equation of the calcium hydroxide. Although this lowering of the pH value in the unhairing solution has a beneficial effect in decreasing the swelling of the hides or skins, it has been found that too large a reduction in pH value is to be avoided in that this results in a slowing of the unhairing action. Accordingly, in some instances, it may be desirable to provide additional alkalinity in the unhairing solution so as to prevent an excessive drop in the pH.

Insofar as the selection of the phosphorus and sulphur containing compound which is used is concerned, it has been found that it is preferable if the atomic ratio of sulphur to phosphorus in the compound is not appreciably less than about 5:2. Although satisfactory results are obtained, insofar as the speed and degree of unhairing and the condition of the hair are concerned, when using phosphorus and sulphur compounds having a lower sulphur to phosphorus ratio, such as P 8 the hydrolysis of such lower ratio compounds frequently results in the formation of phosphine in the reaction. The formation of this material in the unhairing reaction is desirably avoided. It is for this reason that it is preferred to use the phosphorus and sulphur containing compounds having a sulphur to phosphorus ratio in excess of about 5 :2.

It is believed that those skilled in the art will be able to select readily phosphorusand sulphur-containing compounds, having the preferred sulphur to phosphorus ratio, which will be effective in the unhairing of hides and skins. Exemplary of suitable inorganic phospho-sulphur compounds containing at least one P-S bond which may be used are the phosphorus sulfides, such as phosphorus pentasulfide, (P 8 or P 5 phosphorus polysulfides such as P 8 the various alkali metal thiophosphates, such as the sodium thiophosphates, to mention only a few. Additionally, it is to be understood that compounds such as the phosphorus sulfides may be used in conjunction with an alkali metal hydroxide so as to form in situ, the corresponding alkali metal thiophosphates, in the unhairing solution.

As has been indicated above, the addition of an alkaline mateiral such as sodium hydroxide or potassium hydroxide, in combination with the phosphorus sulfide, may be useful in some instances to prevent an excessive lowering of the pH of the unhairing solution. It is to be further appreciated that the reduction in pH of the hairing solution also can be compensated for by combining the phosphorus and sulphur compound, such as phosphorus sulfides, with a small amount of an alkali metal sulfide, such as sodium sulfide. In this instance, the amount of alkali metal sulfide used will, of course, be small, as compared to the amount of phosphorus sulfide used. Additionally, small amounts of an alkali metal sulfhydrate, such as sodium sulfhydrate, may also be combined With the phosphorus sulfide. This material acts as a sharpening agent in the unhairing solution, as well as contributing to the pH. Inasmuch as the purpose of combining the sulfide and/or sulfhydrate with the phosphorus sulfide is not to efl ect the major amount of hair removal, the amount of these materials present will be less than that which would cause any damage to the hair which is removed.

In order to better understand the type of inorganic phospho-sulphur compound containing at least one P-S bond which may be used in the practice of the present invention, an explanation of the reaction mechanisms taking place in the unhairing solution will be attempted. It is to be appreciated, however, that the following is being set forth merely as a proposed theory as to the chemical reactions which take place and is not to be interpreted as a limitation in the present invention. It is believed that where the phosphorus and sulphur containing compound is added to the lime slurry, this compound hydrolyzes to form sulfhydrate ions and phosphate ions. Where the phosphorus and sulphur containing compound used is a phosphorus sulfide, such as phosphorus pentasulfide, it is believed that the alkali metal or alkaline earth metal thiophosphate is first formed and that this in turn hydro lyzes to the sulfhydrate ions and phosphate ions. Thereafter, the sulfhydrate ions are further hydrolyzed to form hydrogen sulfide, which material, it is believed, efiects the actual unhairing. Inasmuch as the hydrogen sulfide is acidic, its formation results in an increase in the acidity of the unhairing solution thus causing the dissolving of additional calcium hydroxide with a resulting decrease in the pH due to the common effect, as has been indicated hereinabove. Thus, by adding an alkaline material such as caustic soda or sodium sulfhydrate with the phosphorus sulfide, an additional source of alkalinity is provided in the unhairing solution to counter-act this increase in acidity and maintain the pH at a constant level. Accordingly, the inorganic phospho-sulphur compounds containing at least one P-S bond which may be used in the present invention may be thought of as those phosphorus and sulphur compounds, preferably having a sulphur to phosphorus ratio not less than about 5:2, which in a lime slurry or solution will ultimately hydrolyze to form hydrogen sulfide. These phosphorus and sulphur containing compounds may be used by themselves or in conjunction with a material which will be hydrolyzed to provide a source of alkalinity in the lime solution or slurry. It is believed that in addition to the specific phosphorus and sulphur containing compounds which have been set forth hereinabove, other similar compounds will become apparent to those skilled in the art.

As has been pointed out hereinabove, the inorganic phospho-sulphur compounds containing at least one P-S bond used in the present invention may be a phosphorus sulfide such as phosphorus pentasulfide, phosphorus polysulfide, a sodium thiophosphate or a combination which will yield sodium thiophosphate in situ, such as a mixture of phosphorus pentasulfide and caustic soda. Exemplary of a sodium thiophosphate which may be used in the present method is one prepared by the reaction of phosphorus pentasulfide (P 5 and sodium hydroxide in a water solution. This reaction may be carried out by adding the phosphorus pentasulfide to a concentrated aqueous solution of caustic soda at such a rate that the temperature of the reaction is maintained at about degrees centigrade, the reaction being moderately rapid and exothermic in nature. When the desired concentra= tion of phosphorus pentasulfide is obtained, the sodium thiophosphate product may be crystallized from the reaction mixture by cooling it to a temperature of about 30 degrees centigrade provided the NaOH cencentration has been sufficiently high. The mother liquor which remains after the removal of the crystals may then be reused in additional runs. The white to cream-colored crystals obtained can be purified by recrystallization from Water to give the final sodium thiophosphate product which melts in its water of crystallization at about 65 degrees cen tigrade and which decomposes with the evolution of hydrogen sulfide at about to decrees centiurnde.

In unhairing animal skins and hides in accordance with the present invention, a solution or slurry of lime is used. Inasmuch as the lime is continually being used during the unhairing process, it is preferable that a lime slurry be used so that solid lime is always available to go into solution and replenish the lime which has been used. While, obviously, this result could be obtained :by continuously adding lime to the unhairing solution at a rate to correspond to the rate at which the lime is being used, a simpler and hence preferable, method is to use a lime slurry. Generally, the slurry used will contain about 6 percent of lime, based on the total weight of the hides being unhaired. This amount of lime, of course, includes both the lime which is in solution and that which remains in the solid state suspended in the solution. Although a slurry containing about 6 percent by weight of lime is preferred, it should be appreciated that lime slurries containing either greater or lesser amounts of lime may be used with satisfactory results.

To the above lime slurry is added an inorganic phosphosulphur compound containing at least one P-S bond, as has been described in detail hereinabove. The amount of this compound added, whether phosphorus sulfides or sodium thiophosphates or other similar compounds will generally be within the range of about 0.1 to about 10 percent by weight of the hides or skins being treated. Excellent results have been obtained in the practice of the present invention when the amount of phosphorus and sulphur containing compound used is within the range of about 1 to 2 percent by weight of the hides being treated. Accordingly, this amount of phosphorus and sulphur containing compound is preferred.

Where an alkaline material, such as caustic soda, or an alkaline forming material, such as sodium sulfhydrate, is used in conjunction with the phosphorus and sulphur containing compound in the practice of the present invention, these materials will generally be present in an amount by weight not substantially in excess of the phosphorus and sulphur containing compound. Preferably, and particularly where an alkali yielding material such as sodium sulfhydrate is used, the weight amounts will be substantially less than the amount of phosphorus and sulphur containing compound. The above weight amounts of alkali, such as caustic soda, and phosphorus and sulphur containing compound, such as phosphorus pentasulfide, will provide a mole ratio of caustic soda to phosphorus pentasulfide of about 6:1, which mole ratio has been found to give particularly good results in the practice of the present inveu tion.

In effecting the unhairing of animal hides and skins in accordance with the present invention, the hides or skins are preferably subjected to an aqueous soaking operation prior to the time when they are contacted with the unhairing slurry or solution. As has been indicated hereinabove, one of the pre-tanning treatments which animal hides and skins customarily undergo is that of curing. This treatment customarily involves treating the hides with rock salt so as to prevent decay prior to processing into leather. Before the remaining pre-tanning treatments can be carried out on the hides, it is necessary that the salt be removed. This removal of the salt as well as dirt and other soluble matter from the hide is one function of the soaking operation. A second and equally important function is that of restoring water to the hides so as to return them to a condition which is, as nearly as possible, equivalent to their condition at the time they were on the live animals.

The soaking operation prior to unhairing may be carried out using water as the soaking media. Generally, it has been found to be preferable if small amounts of an alkali metal sulfhydrate and/or an alkali metal tetrasulfide are added to the soaking water. Where these materials are added to the soaking solution, it is found that a preliminary loosening of the hair is achieved during the soaking operation. Generally, it has been found that the alkali metal sulfhydrate, such as sodium sulfhydrate and the alkali metal tetrasulfide such as sodium tetrasulfide may be added to the soaking water in amounts up to about 2 percent by weight of the hides being soaked. Such amounts of these materials have been found to be beneficial in aiding the unhairing process and have not been found to detrirnentally affect the condition of the hair which is removed. It is to be understood, of course, that the use of these sulphur containing additives is not essential in the present process, but is preferred.

In general, the time required for soaking of the hides prior to unhairing will be upwards to about 24 hours, whether or not additives such as sodium sulfhydrate and/ or sodium tetrasulfide are used. While, obviously, the minimum amount of soaking solution used is that which will provide for a thorough wetting of the hides, there has not been found to be any critical maximum amount. In general, a weight ratio of soaking solution to hides of about 4:1 have been found to give very good results and for this reason is preferred. The temperatures utilized in the soaking operation, again, have not been found to be critical, although, it will be appreciated that at higher temperatures the soaking will proceed more rapidly. For convenience, however, the soaking operation is generally carried out at about room temperature, i.e., about 70 degrees Fahrenheit. Higher or lower temperatures can be used, however, in this operation without detrimental effects. During the soaking operation, the hides or skins may be agitated or paddled. The frequency and amount of such agitation or paddling will be determined by whatever practice is normally followed in ordinary soaking procedures.

Once the soaking operation has been completed, the hides or skins are placed in the unhairing solution. Generally, the unhairing is carried out in paddles although, if desired, vats or still pits may be used. The lime slurry which forms the basis for the unhairing solution is made up in the concentration as set forth hereinabove. To the lime slurry is added the desired inorganic phospho-sulphur compound containing at least one P-S bond in amount heretofore indicated. Where an alkaline material such as sodium hydroxide is added in conjunction with a phosphorus sulfide so as to form sodium thiophosphate in situ in the lime slurry, sutficient time is allowed to elapse after the addition of the caustic soda and phosphorus sulfide to allow for the dissipation of the initially high alkaline condition which obtains in the solution, before adding the hides or skins to be unhaired. The temperature at which the unhairing operation is carried out has not been found to be critical. Accordingly, the unhairing solution is generally at about room temperature, i.e., about 70 degrees to degrees Fahrenheit. The amount of solution used -in the unhairing is generally about the same as that used for the soaking operation, i.e., a weight ratio of solution to hides of about 4: 1. The hides or skins to be unhaired are then combined with the unhairing solution in the paddle where they remain until the hair has been sufiiciently loosened. The time required may be variable, depending on the hides, solution strength, and so forth. Generally, however, periods of about '1-8 to 24 hours should be sufiicient to effect substantially complete unhairing of the hides. During the time the hides are in contact with the unhairing solution, they are agitated or paddled so as to elfect a more intimate contact between the hides and the solution. As in the case of soaking operation, the time and the nature of paddling will be those which are suitable. After the hides or skins are removed from the unhairing solution, they are passed through conventional unhairing equipment wherein substantially all the hair on the hide is removed. By use of the above procedure, substantially complete unhairing of the hides is obtained in a comparatively short time, as compared to prior art methods, with substantially no damage to the hair.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given.

Example 1 Grams of Hair Sample Phosphorus Final pH Unhalring Conditions Peutasulfide 0 12. 5 Poor 1. 25 12. 4 Fair 2. 50 12. 3 G00 10 10.5 Poor Example 2 The 'procedure of Example -1 is repeated with the exception that varying quantities of sodium sulfhydrate and/or phosphorus pentasul-fide are added to the lime solution. The unhairing of the hide pieces is again evaluated after 24 hours. Using this procedure, the following results are obtained:

7 Example 3 Sample Grams of Final pH Unhairing Hair Pass Added Conditions 12.4 Fair Good. 2. 12. 3 Good Do. 5.0 12.3 do Do.

From the above results it is seen phosphorus pentasulfide is effective in increasing the unhairing efficiency of a lime slurry. It is further seen that unhairing with phosphorus pentasulfide is effected when using either a water soak prior to the unhairing process or an aqueous solution containing sodium sulfhydrate and sodium tetrasultide, although the use of the latter soak solutions does result in a somewhat increased effectiveness for the phosphorus pentasuliide. It is also to be noted from Example 1 above that when a large quantity of phosphorus pentasulfide is used, without the addition of an alkaline material such as sodium sulfhydrate, there is an appreciable drop in the final pH of the unhairing solution with a resulting decrease in unhairing efficiency.

Example 4 The following illustrates the preparation of a sodium thiophosphate for use in the unhairing process of the present invention.

Phosphorus pentasulfide (P 8 is reacted with a 10 percent excess of a strong caustic solution containing percent by weight NaOH. The reaction is moderately rapid and exothermic, and the temperature during the reaction is held at about 70 degrees centigrade by controlling the rate of addition of the P 8 to the caustic solution. From this reaction, a concentration of sodium thiophosphate between 80-85 grams per 100 cubic centimeters of water is obtained. Cooling this solution to degrees centigrade, effects crystallization of a heavy crop of crystals. These crystals are separated from the mother liquor and solid caustic soda and P 8 are again added to the mother liquor to prepare a second batch. This procedure is repeated for a total of six times to yield a total of 3 pounds of crude crystals. These crystals, after being removed from the mother liquor by filtration, are dried between filter papers and are cream-colored. The crystals are found to be stable at room temperature and melt in their water of crystallization at 65 degrees centigrade. The crystals decompose with the evolution of hydrogen sulfide when they are heated to a temperature within the range of about -90 degrees centigrade. The solubility of these crystals at 28 degrees centigrade is about 110 grams per 100 cubic centimeters of water. At 10 degrees centigrade, the solubility is 60 grams per 100 cubic centimeters of water. Analysis of these crystals shows a phosphorus content of 8.29 percent and a sulphur content of 22.17 percent.

Example 5 The procedure of Example 1 is repeated with the exception that varying quantities of sodium thiophosphate,

prepared in the manner set forth in Example 4, are added to the lime solution. The hide pieces are evaluated for unhairing after 24 hours. Using this procedure, the following results are obtained:

l Grams of Hair Sample Sodium Thio- Final pH Unhalring Conditions phosphate Example 6 The procedure of Example 5 is repeated with the exception that varying quantities of sodium sulfhydrate and/ or sodium thiophosphate are added to the lime solution. The unhairing of the hides is then evaluated after 24 hours. Using this procedure, the following results are obtained:

The procedure of Example 3 is repeated, using a soak of sodium sulfhydrate and sodium tetrasulfide, with the exception that varying quantities of sodium thiophospbate, prepared in the manner set forth in Example 4, are added to the lime solution. Evaluation of unhairing is made after 24 hours. Using this procedure, the following results are obtained:

Grams of Hair Sample of Na Thio- Final pH Unhalring Condition phosphate 0 12. 4 Poor Good. 2. 5 12.4 Fair Do. 5.0 12. a Good i Do. 10 12. 4 Very good Do. 20 12.4 lo...-. D0. 40 12.4 .do Fair.

From the above results it is clearly seen that sodium thiophosphate, like phosphorus pentasulfide, is effective in increasing the unhairing efficiency of a lime slurry or solution. It is further seen that the sodium thiophosphate may be used by itself, or in conjunction with an alkaline material such as sodium sulfhydrate in effecting the unhairing. In this regard, it is to be noted in Example 6 that where the amount of sodium sulfhydrate used in conjunction with the sodium thiophosphate is too great, some hair damage does result. The above examples further illustrate that the sodium thiophosphate may be used for unhairing after either a conventional water soak or the preferred soak of an aqueous solution containing sodium sulfhydrate and sodium tetrasulfide.

A series of runs is made using 500 gram samples of hides which have been water soaked in the manner set forth in Example 1. Following the soaking operation, the hide samples are placed in 2000 milliliters of water at degrees Fahrenheit, and varying quantities of sodium thiophosphate and other alkaline materials are added. The unhairing effectiveness in each case is observed after 24 hours. Using this procedure, the following results are obtained.

Sample Materials Added Firfial Unharing 1 40 g. Trisodium phosphate, g. 10.7 Fair.

Nn thiophosphate.

2 40 g. N22200:, 10 g. Na thio- 10. 7 D0.

phosphate.

3 40 g. Trisodiurn phosphate, 40 g. 11.0 Good.

Ammonium hydroxide, 10 g. N a thiophosphate.

4 40 g. NagCOs, 40 g. NHAOH, 10 g. 10. 9 Do.

Na thiophosphate.

Example 9 Hides to be unhaired are soaked in water overnight at 70 degrees Fahrenheit and then divided into samples of 500 grams each. Unhairing slurries are made containing 6 percent by weight of lime, based upon the weight of the hides to be unhaired, plus additional substituents in the amounts as indicated hereinbelow. The additional substituents may be added individually but it is often preferred to premix them, either with or without the lime, and add them as a dry mix or sometimes, even as a solution or slurry. The percentages given are based on the weight high concentration of caustic soda in the solution, resulting in the absorption of caustic soda by the hide. This indicates desirability of allowing some time to elapse after mixing the phosphorus pentasulfide and caustic soda with the lime slurry and before the hides are put into this slurry. This is further illustrated by the following example.

Example 10 In this experiment, the hides are soaked in water according to the procedure used in the preceding example. To the unhairing slurry, containing 6 percent by weight of lime, based on the weight of the hides to be unhaired and 2 percent by weight of phosphorus pentasulfide, based on the weight of the hides, are added varying amounts of caustic soda, as indicated hereinbelow. A series of runs was made wherein different times elapsed between the addition of the phosphorus pentasulfide and caustic soda to the slurry and the addition of the hides to the slurry. After 24 hours, the amount of unhairing on the hides was noted and, additionally, the amount of swelling of the hide was evaluated. The amount of swelling which takes place is rated on a scale of from 0 to 2, using the amount of swelling occurring when sodium sulfhydrate is used in the unhairing solution as a standard. In this evaluation, 0 is comparable to sodium sulfhydrate; (l) is slightly more swollen than sodium sulfhydrate; and (2) is considerably more swollen than sodium sulfhydrate. Using this procedure, the following results are obtained:

of hides used. The sodium thiophosphate added to the lime slurry in some of these runs is prepared in accordance with the procedure set forth in Example 4. The samples of hides were then placed in the unhairing solution and the effects of unhairing were noted after 18 hours.

Using this procedure, the following results are obtained:

From these results, it is clearly seen that in those runs wherein an appreciable amount of time is allowed to elapse between the addition of the caustic soda and phosphorus pentasulfide and the addition of the hide, there is very little swelling of the hide. In contrast, where there is only a short lapse of time before the hides are added to the Materials, percent Sample Pass NaOH NaSH Unhairing Hair Condition In the above runs, it is to be noted that when using the high concentrations of phosphorus pentasulfide and sodium thiophosphate, a very slight amount of hair damage did occur. Additionally, in runs 7, 8, and 9, there was swelling in the hides, which swelling was caused by the unhairing slurry, considerably more swelling of the hides result. These runs are seen to emphasize the fact that it is preferable that where caustic soda is used in combination with phosphorus pentasulfide in the unhairing slurry, permitting some time to elapse before the hides are added 1 l to the slurry aids in preventing any undue swelling of the hides.

Example 11 Materials Mole ratio Sample NaO HI Unhairing P185, NaOH, NsSH, P155 percent g. percent Good.

2. Fair. 2. 0 Good. 2. 0 Do. 2. 0 Do. 2. 0 Very good. 2. 0 Do. 2. 0 Do. 2. 0 Do.

Example 12 In this example, the hides are soaked overnight in a soak solution containing 2 percent by weight of sodium sulfhydrate and 0.25 percent by weight sodium tetrasulfide, the weights being based on the weight of stock soaked. The weight ratio of the soaking solution to the hides being soaked is 4:1 and the temperature of the soaking solution during the soaking period is room temperature, 70 degrees Fahrenheit. T he next day the hides are placed in a lime slurry containing 6 percent by weight of lime, based on the Weight of the stock, as well as varying amounts of phosphorus pentasulfide and caustic soda, as indicated hereinbelow, proportions given being based on the weights of the hides. The lime and the phosphorus pentasulfide-caustic soda mixture were mixed for a period of one hour prior to the addition of the hides. The sulfhydrate-soaked hides were rinsed with one change of water for 10 minutes prior to placing them in the lime slurry. The amount of unhairing resulting was evaluated at the end of 24 hours. Using this procedure, the following results are obtained:

From the above results, it is seen that sodium pentasulfide-caustic soda containing system shows better unhairing at the end of 24 hours than does the sulfhydrate control. In no instances in these runs was there any evidence of hair damage nor was there any excessive swelling of the hide.

Example 13 In this example, the phosphorus sulfide compound used in the unhairing solution is a phosphorus polysulfide approximating the formula P 8 In this example, 4000 grams of cattle hides were soaked in a soaking solution containing 2% by weight of sodium sulfhydrate and 0 .25

1'2 percent by weight sodium tetrasulfide, based on the weigh of the hides being soaked. The weight ratio of soaking solution to hides was 4: 1. After soaking overnight, the hide pieces were divided into 500 gram samples for the unhairing tests. The soak solutions were then drained and the hide pieces were transferred into the unhairing bath. The unhairing baths contain 6 percent by weight of lime, based on the weight of the hides and, also based on the weight of the hides, the weights of caustic soda and P 8 indicated hereinbelow. The caustic soda and lime were mixed together and then the P 8 were added. These reagents were allowed to mix on a tumbler for 1 hour prior to the addition of the hides. The results of the unhairing were evaluated after 24 hours in the unhairing bath. Using this procedure, the following results were obtained:

Materials Initial Final Sample Unhairing pH pH NQLOH, P4324,

g. percent 1.6 i 0.5 12.5 12.5 3. 3 1. 0 12. 5 12. 5 5. 0 l. 5 12. 5 12. 5 6.7 2.0 o 12.4 12.4 3. 3 0. 5 Very good... 12. 5 12. 5 6. 7 1. 0 d 12. 5 12. 5

13. 4 i Z. 0 12. 6 l2. 5

Example 14 This example is carried out in order to evaluate the present invention on a commercial scale. In this run, 20 sides of green salted cattle hides, weighing 500 pounds, were placed in a paddle with water at degrees Fahrenheit. The sides were floated for onehalf hour, paddled for 5 minutes and then allowed to rest for an additional hour. At the end of this period, the stock was pulled and fleshed.

The paddle was then drained, refilled with water at degrees Fahrenheit and to this water was added 2 percent sodium sulfhydrate (based on the stock weight) and 0.25 percent sodium tetrasulfide (based on the stock weight). These percentages are of the commercial products containing 74 percent sodium sulfhydrate and 70 percent sodiunitetrasulfide, respectively. The pH of this soak solution was 8.6 and the temperature was 72 degrees Fahrenheit. The fleshed sides were then placed in the solution, paddled for 10 minutes, rested for one-half hour and then paddled for an additional 10 minutes. The stock remained in this solution for 24 hours. At the end of this time, the paddle was drained and the stock was refloated with water at degrees Fahrenheit, paddled for 5 minutes, drained, and then floated again with water at 80 degrees Fahrenheit.

In a barrel, separate from the paddle containing the hides, 10 pounds and 13 ounces of caustic soda were dissolved and diluted with Water to a solution of 25 gallons. Into the caustic soda, 10 pounds of phosphorus pentasulfide were slowly added. The solution was mixed so as to dissolve the phosphorus pentasulfide. The reaction between the phosphorus pentasulfide and the caustic soda raised the temperature of the mixture in the barrel about 10 degrees Fahrenheit. This caustic soda-phosphorus pentasulfide solution was then aged in the barrel for about 1 hour prior to being added to the paddle containing the hides.

Just prior to the addition of the caustic soda-phosphorus pentasulfide solution, 30 pounds of lime (6 percent by weight based on the weight of the stock) were added to the paddle and the hides were paddled in this lime solution for 5 minutes. Thereafter, the solution of phosphorus pentasulfide and caustic soda was added and the paddling was continued for one hour. The initial pH of the lime bath containing the phosphorus pentasulfide and caustic soda was 12.5 and the initial temperature was 80 degrees Fahrenheit. At the end of an hour of paddling, much of the hair on the hides was beginning to loosen and there was some formation of scud on the surface. The hides were then allowed to rest in the paddle for 2 hours followed by an additional paddling for minutes. The paddle was then allowed to rest overnight. The next day, after 18 hours in the lime, the stock was removed for unhairing. The temperature at the time of pulling the stock from the paddle was 70 degrees Fahrenheit and the pH was 12.5.

The hides were then unhaired on a standard unhairing machine without any adjustment of the machine from the regular production stock runs. The unhairing was substantially complete and was equivalent to that obtained from the regular process in the tannery wherein a normal 2 day process in the lime was used. After unhairing, the stock was lime split and was placed in a regular production pack for bate, pickle and tan operation. All of the sides were made into a vegetable ret-an shoe upper leather of a relatively soft type. The leather was evaluated and found comparable to regular production leather in all respects. The sides were smooth, mellow, free of draw and well let-out in the neck, flanks and belly.

From this example, it is seen that substantially complete unhairing can be obtained on a commercial scale without hair damage and in an appreciably shorter time than that generally required for such operation at the present time.

In the preceding examples, it will be appreciated, one also utilizes dry mixes of the various substituents described, instead of adding them individually, and obtains substantially the same results.

While there have been described various embodiments of the invention, the compositions and method described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible and it is further intended that each element or component recited in any of the following claims is to be understood as referring to all equivalent elements or components for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method for unhairing animal hides and/or skins which comprises contacting the hides and/or skins to be unhaired with an aqueous solution containing an unhairing amount of inorganic phospho-sulphur compound containing at least one P-S bond which compound is hydrolyzable in the solution to form sulfhydrate ions and maintaining the hides and/or skins in this solution for a sufi'icient period of time to effect substantially complete loosening of the hair thereon.

2. A method for unhairing animal hides and/or skins which comprises contacting the hides and/or skins to be unhaired with an aqueous suspension of lime containing an unhairing amount of an inorganic phospho-sulp-hur compound containing at least one P-S bond, which com pound is hydrolyzable in the solution to form sulfhydrate ions and maintaining the hides and/ or skins in this solution for a sufiicient period of time to effect substantially complete loosening of the hair thereon.

3. The method claimed in claim 2 wherein the inorganic phospho-sulphur compound containing at least one P-S bond is a compound selected from the group consisting of phosphorus pentasulfide, polymeric phosphorus sulfides and alkali metal thiophosphates.

4. The method as claimed in claim 3 wherein the compound is phosphorus pentasulfide.

5. The method as claimed in claim 4 wherein the phosphorus pentasulfide is present in the solution in the amount within the range of about 0.1 to 10 percent by weight of the hides and/or skins being unhaired.

6. A method for unhairing animal hides and/or skins which comprises contacting the hides and/or skins to be unhaired with an aqueous suspension of lime containing an unhairing amount of an inorganic phospho-sulphur compound containing at least one P-S bond which compound is hydrolyzable in the solution to form sulfhydrate ions and a material which is hydrolyzed in the solution to provide a source of alkalinity, the amount of said latter material being suflicient to maintain the pH of the solution substantially constant, and maintaining the hides and/ or skins in the solution for a sufficient period of time to efiect substantially complete loosening of the hair thereon.

7. The method as claimed in claim 6 wherein the inorganic phospho-sulphur compound containing at least one P-S bond is a compound selected from the group consisting of phosphorus pentasulfide, polymeric phosphorus sulfides, and alkali metal thiophosphates and the alkalinity supplying material is an alkali metal hydroxide.

8. The method as claimed in claim 7 wherein the inorganic phospho-sulphur compound containing at least one P-S bond is phosphorus pentasulfide and the alkalinity supplying material is sodium hydroxide.

9. The method as claimed in claim 8 wherein the phosphorus pentasulfide is present in an amount within the range of about 0.1 to about 10 percent by weight of the hides and/ or skins being unhaired.

10. The method as claimed in claim 9 wherein the mole ratio of sodium hydroxide to phosphorus pentasulfide is in the range of about 5:1 to about 8: 1.

11. A method for unhairing animal hides and/or skins which comprises soaking the hides and/or skins to be unhaired in an aqueous solution containing an alkali metal sult'hydrate in an amount within the range of about 0 to about 2 percent by Weight of the hides and/ or skins, and an alkali metal tetrasulfide in an amount within the range of about 0 to about 2 percent by weight of the hides and/or skins, contacting the hides and/or skins after soaking with an aqueous suspension of lime containing an unhairing amount of an inorganic phosphosulphur compound containing at least one P-S bond which is hydrolyzable in the solution to form sulfhydrate ions and maintaining the hides and/or skins in this solution for a suflicient period of time to eifect substantially com-plete loosening of the hair thereon.

12. The method as claimed in claim 11 wherein the aqueous suspension of lime also contains a material which is hydrolyzed in the solution to provide a source of alkalinity.

13. The method as claimed in claim 12 wherein the inorganic phospho-sulphur compound containing at least one P-S bond is a compound selected from the group consisting of phosphorus pentasulfide, polymeric phosphorus sulfides, and alkali metal thiophosphates, and the material providing a source of alkalinity is an alkali metal hydroxide.

14. The method as claimed in claim 13 wherein the inorganic phospho-sulphur compound containing at least one P-S bond is phosphorus pentasulfide and the material providing a source of alkalinity in the solution is sodium hydroxide.

15. The process as claimed in claim 14 wherein the phosphorus pentasulfide is present in the solution in an amount within the range of about 0.1 to about 10 percent by weight of the hides and/ or skins.

16. The method as claimed in claim 15 wherein the mole ratio of sodium hydroxide to phosphorus pentasulfide is within the range of about 5:1 to about 8:1.

References Cited UNITED STATES PATENTS 2,033,913 3/1936 Fiske et al. 8-94 2,067,628 l/l937 Fiske et a1. 8-94 2,229,420 l/194l Neu-gebauer 894.l6 2,169,147 8/1939 Iaeger 8-94.16

(Other references on following page) 15 16. UNITED STATES PATENTS Matthews: Application of Dyestuffs, pages 72, 74, 89, 2 174 497 9 1 3 Hill g 94 15 117 and 118, Pub. 1920 by John Wiley and 8011s, N,Y.C. 2,160,232 4/1939 Schlaeger 23 108 Von Wazer: Phosphorus and Its Compound, vol. 1, 2,272,617 2/1942 Cox 23--108 pages 291-294 and 824826, published by Interscience FOREIGN PATENTS 1958- 378,425 8/1907 France- J. TRAVIS BROWN, Primary Examiner.

OTHER REFERENCES ABRAHAM H. WINKELSTEIN, Examiner.

Wilson: The Chemistry of Leather Manufacture, pages 10 266-270, Pub. 1928, by the Chem. Catalog. Co. Inc., DONALD LEVY, Assistant Examiner. 

1. A METHOD FOR UNHAIRING ANIMAL HIDES AND/OR SKINS WHICH COMPRISES CONTACTING THE HIDES AND/OR SKINS TO BE UNHAIRED WITH AN AQUEOUS SOLUTION CONTAINING AN UNHAIRING AMOUNT OF INORGAINC PHOSPHO-SULPHUR COMPOUND CONTAINING AT LEAST ONE P-S BOND WHICH COMPOUND IS HYDROLYZABLE IN THE SOLUTION TO FORM SULFHYDRATE IONS AND MAINTAINING THE HIDES AND/OR SKINS IN THIS SOLUTION FOR A SUFFICIENT PERIOD OF TIME TO EFFECT SUBSTANTIALLY COMPLETE LOOSENING OF THE HAIR THEREON. 